Evolution of the semiconductor manufacturing industry is placing ever greater demands on yield management and, in particular, on metrology and inspection systems. Critical dimensions continue to shrink. Economics is driving the industry to decrease the time for achieving high-yield, high-value production. Minimizing the total time from detecting a yield problem to fixing it determines the return-on-investment for a semiconductor manufacturer.
Fabricating semiconductor devices, such as logic and memory devices, typically includes processing a semiconductor wafer using a large number of fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a photoresist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing (CMP), etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and, thus, higher profits. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits (ICs). However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices.
Semiconductor technologies have created a high demand inspection of specimens within the nanometer scale. Micrometer and nanometer scale inspection is often done with charged particle beams that are generated and focused in charged particle beam devices. Examples of charged particle beam devices are electron microscopes, electron beam pattern generators, ion microscopes, and ion beam pattern generators. Charged particle beams, in particular electron beams, can offer superior spatial resolution compared to photon beams due to their short wavelengths at comparable particle energy.
One such inspection technology includes electron beam based inspection systems. Electron beam imaging systems typically use an electron beam column to scan an electron beam across a region of a substrate surface to obtain image data. An example of an electron beam based inspection system is a scanning electron microscope (SEM). SEM systems may image a surface of a sample by scanning an electron beam over a surface of a sample and detecting secondary electrons emitted and/or scattered from the surface of the sample. Typical SEM systems include a Wien filter located within the electron-optical column of the SEM and positioned above the sample for the purposes of deflecting the secondary electrons to a secondary electron detector. The utilization of such a Wien filter may cause transverse chromatic aberration in the primary beam.
These electron beam based inspection systems, including SEMs, are becoming increasingly relied upon for inspection of devices formed in semiconductor fabrication. Microscopes that utilize electron beams to examine devices may be used to detect defects and investigate feature sizes as small as, for example, a few nanometers. Therefore, tools that utilize electron beams to inspect semiconductor devices are increasingly becoming relied upon in semiconductor fabrication processes.
The SEM generates a primary electron (PE) beam illuminating or scanning a specimen. The primary electron beam generates particles like secondary electrons (SE) and/or backscattered electrons (BSE) that can be used to image and analyze the specimen. Many instruments use either electrostatic or compound electric-magnetic lenses to focus the primary electron beam onto the specimen. In some cases, the electrostatic field of the lens simultaneously collects the generated particles (SE and BSE) that enter into the lens and are guided onto a detector. If uniform high efficiency electron collection and detection is required, the secondary and/or backscattered particles must be separated from the primary beam. In such a case, the detection configuration can be designed completely independent from the PE optics design. If uniform high efficiency electron collection and detection is required, the secondary and/or backscattered particles must be separated from the primary beam, such as using a beam separator including magnetic deflection fields or a Wien filter element.
The SEM can include a beam separator having one or more electrostatic deflectors for deflecting a beam of primary electrons away from an optical axis normal to the substrate, or for redirecting a deflected beam into the optical axis. Electrostatic deflectors apply a voltage to multiple electrodes, thereby generating an electric field for deflecting the beam. For example, a symmetric quadrupole may be used, in which four electrode plates are spaced 90 degrees apart for deflection of the beam in either an X-direction or a Y-direction. For example, voltages +Vx and −Vx can be applied to the first and third electrodes respectively (the first and third plates opposed on the X-axis). Voltages +Vy and −Vy can be applied to the second and fourth electrodes respectively (the second and fourth plates opposed on the Y-axis).
The beam separator can introduce dispersion of the primary beam and can limit the attainable resolution. One type of Wien filter, an unbalanced type known as achromatic Wien filter can be used to avoid PE beam dispersion. However, these devices typically result in aberrations which can impair spot size and the spot resolution in inspection applications using large beam currents and beam diameters.
Thermally-induced beam drift also can occur. Previous techniques used a calibration scheme to detect drift in the electron beam. However, this calibration scheme needs constant beam position calibration, which makes it difficult to run long inspection or review jobs without drift compensation. This can cause negative effects on throughput.
Therefore, new techniques to address thermally-induced beam drift are needed.